(Sensitized) Photolysis of Diazonium Salts as a Mild General Method for the Generation of Aryl Cations. Chemoselectivity of the Singlet and Triplet 4-Substituted Phenyl Cations

• Published in: Journal of organic chemistry Vol. 70; no. 2; pp. 603 - 610
• Main Authors: Milanesi, Silvia, Fagnoni, Maurizio, Albini, Angelo
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 21-01-2005
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Kinetics and mechanisms; Noncondensed benzenic compounds; Organic chemistry; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Preparations and properties; Reactivity and mechanisms; Substituent effect; Arylation; Xanthone; Organic cation; Chemoselectivity; Singlet state; Allylic compound; Fragmentation; Triplet state; Photolysis; Biphenyl derivatives; Acetonitrile; Reaction mechanism; Benzenic compound; Aromatic compound; Sensitization; Diazonium compounds; Solvent; Ethylenic compound; Excited state; Nucleophile
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo048413w

The photolysis of a series of 4-X-benzenediazonium tetrafluoroborates is studied in MeCN. Loss of nitrogen occurs from the singlet excited state with X = H, t-Bu, and NMe2 and leads to the singlet aryl cation. This adds to the solvent yielding the corresponding acetanilides. With other substituents, ISC competes with (X = Br, CN) or overcomes (X = COMe, NO2) fragmentation and the aryl cation is formed in part or completely in the triplet state. In neat MeCN, this either abstracts hydrogen from the solvent (in most cases inefficiently) or undergoes intersystem crossing to the more stable singlet that reacts as above. In the presence of π nucleophiles (allyltrimethylsilane or benzene), the triplet aryl cation is efficiently trapped giving substituted allylbenzenes and biphenyls, respectively. By triplet sensitization by xanthone, the triplet cation and the products from it are obtained from the whole series considered. The direct or sensitized photodecomposition of diazonium fluoroborates, substituted with both electron-donating and -withdrawing substituents, in the presence of alkenes and arenes offers an access to an alternative arylation procedure.